Foam sheet of crosslinked polyolefin resin,process for producing the same and pressure-sensitive adhesive tape

ABSTRACT

The present invention provides a crosslinked polyolefin-based resin-extruded foam sheet capable of reducing its thickness while retaining excellent flexibility and heat resistance. The crosslinked polyolefin-based resin foam sheet of the present invention is obtained by feeding a polyolefin-based resin and a thermally degradable blowing agent to an extruder, melting and kneading them, extruding the kneaded material through the extruder into a sheet to form an expandable polyolefin-based resin sheet, and expanding the sheet. Herein, a degree of crosslinking of the crosslinked polyolefin-based resin foam sheet is 5 to 60% by weight, an aspect ratio of a cell (MD average cell diameter/CD average cell diameter) is 0.25 to 1, and the polyolefin-based resin contains 40% by weight or more of a polyethylene-based resin obtained using a metallocene compound containing a tetravalent transition metal as a polymerization catalyst.

TECHNICAL FIELD

The present invention relates to a crosslinked polyolefin-based resinfoam sheet having excellent flexibility and heat resistance and aprocess for producing the same, as well as an adhesive tape using thecrosslinked polyolefin-based resin foam sheet.

BACKGROUND ART

Conventionally, polyolefin-based resin foam sheets have been used in awide range of utility, and examples of this utility may include sealingmaterials for electronic appliances in addition to a base material of anadhesive tape and a base material of a patch. In these utilities, apolyolefin-based resin foam sheet is required to have a reducedthickness, and flexibility.

As such a polyolefin-based resin foam sheet, Patent Document 1 hasproposed a porous sheet with flexibility and mechanical strengthimproved by adding an inorganic filler to a polyolefin-based resin toform a sheet, and stretching this sheet to render it porous.

However, since it is difficult to realize a high expansion ratio of theporous sheet, the sheet has insufficient flexibility. Further, since theporous sheet has not been subjected to crosslinking treatment,stretching treatment can not be performed at a melting point or more ofa polyolefin-based resin constituting a porous sheet; therefore, thereis a problem that distortion is occurred in the porous sheet duringstretching treatment, and the porous sheet is shrunk during use.

In addition, as a polyolefin-based resin foam sheet, Patent Document 2has proposed a crosslinked ethylene-based resin foam which comprises asuper density polyethylene resin having a predetermined density and afoaming agent, and is obtained by expanding a sheet which has beenirradiated with an electron beam of 1 Mrad or less.

However, the crosslinked ethylene-based resin foam has insufficientflexibility and heat resistance. It is considered that the flexibilityis improved by increasing an expansion ratio, but there is a problemthat when the expansion ratio is increased, the resulting foam is thick.

Patent Document 1: JP-B 7-64942

Patent Document 2: JP-B 6-76505

DISCLOSURE OF THE INVENTION

Problems to be Solved by the Invention

The present invention provides a crosslinked polyolefin-based resin foamsheet capable of reducing its thickness while retaining excellentflexibility and heat resistance, a process for producing the same, aswell as an adhesive tape using the crosslinked polyolefin-based resinfoam sheet.

Means for Solving the Problems

The crosslinked polyolefin-based resin foam sheet of the presentinvention is a crosslinked polyolefin-based resin foam sheet obtained byfeeding a polyolefin-based resin and a thermally degradable blowingagent to an extruder, melting and kneading them, extruding it through anextruder into a sheet to form an expandable polyolefin-based resinsheet, and expanding the sheet, wherein a degree of crosslinking of thecrosslinked polyolefin-based resin foam sheet is 5 to 60% by weight, anaspect ratio of a cell is within a predetermined range, and thepolyolefin-based resin contains 40% by weight or more of apolyethylene-based resin obtained using a metallocene compoundcontaining a tetravalent transition metal as a polymerization catalyst.

The polyolefin-based resin constituting the crosslinked polyolefin-basedresin foam sheet is not particularly limited as far as it contains 40%by weight or more of a polyethylene-based resin obtained using ametallocene compound containing a tetravalent transition metal as apolymerization catalyst, and may be only a polyethylene-based resinobtained using a metallocene compound containing a tetravalenttransition metal as a polymerization catalyst, or a resin consisting ofa polyethylene-based resin obtained using a metallocene compoundcontaining a tetravalent transition metal as a polymerization catalyst,and another polyolefin-based resin.

In the polyolefin-based resin, the content of the polyethylene-basedresin obtained using a metallocene compound containing a tetravalenttransition metal as a polymerization catalyst is limited to 40% byweight or more, preferably 50% by weight or more, more preferably 60% byweight or more, particularly preferably 100% by weight. Herein, 100% byweight of the content of the polyethylene-based resin obtained using ametallocene compound means a case where only the polyethylene-basedresin obtained using a metallocene compound is used as thepolyolefin-based resin.

The reasons why the content of the polyolefin-based resin obtained usinga metallocene compound in the polyolefin-based resin is limited to 40%by weight or more will be explained. The first reason is as follows.

A crosslinked polyolefin-based resin foam sheet is produced bystretching a foam sheet in a predetermined direction while expanding orheating as described later. Upon stretching of this foam sheet, cells ofthe foam sheet are stretched in a stretching direction, and cell wallsare brought into close to each other. Therefore, when a resin havingadherability (e.g., ethylene-vinyl acetate copolymer) is used as apolyolefin-based resin, cell walls are adhered and incorporated, and anaspect ratio of a cell in a predetermined range can not be obtained. Onthe other hand, the crosslinked polyolefin-based resin foam sheet of thepresent invention is required to have flexibility.

Thus, by using the polyolefin-based resin containing 40% by weight ormore of the polyethylene-based resin obtained using a metallocenecompound, flexibility is imparted to the polyolefin-based resin withoutincreasing adherability, and an aspect ratio of a cell of thecrosslinked polyolefin-based resin foam sheet is set within apredetermined range, thereby improving a mechanical strength andrendering flexibility excellent.

The second reason is as follows. The polyethylene-based resin obtainedusing a metallocene compound has a narrow molecular weight distributionand, in the case of a copolymer, a copolymer component is introduced inany molecular weight component at an approximately equal ratio.Therefore, a foam sheet can be evenly crosslinked. Further, since a foamsheet is evenly crosslinked, the foam sheet can be evenly stretched, anda thickness of the resulting crosslinked polyolefin-based resin foamsheet can be totally even.

As the polyethylene-based resin obtained using a metallocene compoundcontaining a tetravalent transition metal as a polymerization catalyst,a linear low density polyethylene obtained by copolymerizing ethyleneand a small amount of α-olefin using a metallocene compound containing atetravalent transition metal as a polymerization catalyst is preferable.

Examples of the α-olefin may include propylene, 1-butene, 1-pentene,4-methyl-1-pentene, 1-hexene, 1-heptene, 1-octene and the like.

The metallocene compound generally refers to a compound having astructure in which a transition metal is held by π electron systemunsaturated compounds, a representative of which is abis(cyclopentadienyl)metal complex.

Specific examples of the metallocene compound containing a tetravalenttransition metal in the present invention may include compounds in whichone or two or more cyclopentadienyl rings or analogues thereof arepresent as a ligand for a tetravalent transition metal such as titanium,zirconium, nickel, palladium, hafnium and platinum.

Examples of the ligand may include a cyclopentadienyl ring; acyclopentadienyl ring substituted with a hydrocarbon group, asubstituted hydrocarbon group or a hydrocarbon-substituted metalloidgroup; a cyclopentadienyloligomer ring; an indenyl ring; an indenyl ringsubstituted with a hydrocarbon group, a substituted hydrocarbon group ora hydrocarbon-substituted metalloid group, and the like. In addition tothese π electron system unsaturated compounds, as a ligand, a monovalentanion ligand such as chlorine and bromine or a divalent anion chelateligand, hydrocarbon, alkoxide, arylamide, aryloxide, amide arylamide,phosphide, and arylphosphide may be coordination-bonded to a transitionmetal atom.

Further, examples of a hydrocarbon group substituting at acyclopentadienyl ring may include a methyl group, an ethyl group, apropyl group, a butyl group, an isobutyl group, an amyl group, anisoamyl group, a hexyl group, a 2-ethylhexyl group, a heptyl group, anoctyl group, a nonyl group, a decyl group, a cetyl group, a phenyl groupand the like.

Examples of such a metallocene compound containing a tetravalenttransition metal may includecyclopentadienyltitaniumtris(dimethylamide),methylcyclopentadienyltitaniumtris(dimethylamide),bis(cyclopentadienyl)titanium dichloride,dimethylsilyltetramethylcyclopentadienyl-t-butylamidezirconiumdichloride, dimethylsilyltetramethylcyclopentadienyl-t-butylamidohafniumdichloride,dimethylsilyltetramethylcyclopentadienyl-p-n-butylphenylamidezirconiumchloride,methylphenylsilyltetramethylcyclopentadienyl-t-butylamidehafniumdichloride, indenyltitaniumtris(dimethylamide),indenyltitaniumtris(diethylamide),indenyltitaniumtris(di-n-propylamide),indenyltitaniumbis(di-n-butylamide)(di-n-propylamide), and the like.

The metallocene compound exerts action as a catalyst upon polymerizationof various olefins by changing a kind of a metal and a structure of aligand, and combining with a particular cocatalyst (promoter).Specifically, polymerization is usually performed in a catalyst systemin which methylaluminoxane (MAO), a boron-based compound or the like isadded as a cocatalyst to these metallocene compounds. A ratio of acocatalyst to be used relative to the metallocene compound is preferably10 to 1,000,000 molar times, more preferably 50 to 5,000 molar times.

A method of polymerizing the polyethylene-based resin is notparticularly limited, and examples thereof may include a solutionpolymerization method using an inert medium, a bulk polymerizationmethod substantially free of an inert medium, a vapor phasepolymerization method and the like. A polymerization temperature isusually −100° C. to 300° C., and a polymerization pressure is usually anormal pressure to 100 kg/cm².

In the metallocene compound, active points are even in property and havethe same active degree; therefore, evenness in a molecular weight, amolecular weight distribution, a composition, a composition distributionand the like of a polymer to be synthesized is enhanced. Therefore, apolyolefin-based resin polymerized using these metallocene compounds asa polymerization catalyst has a narrow molecular weight distributionand, in the case of a copolymer, the resin has a characteristic that acopolymerization component is introduced in any molecular weightcomponent at an approximately equal ratio.

Further, examples of a polyolefin-based resin other than apolyethylene-based resin obtained using a metallocene compoundcontaining a tetravalent transition metal as a polymerization catalystmay include a polyethylene-based resin, a polypropylene-based resin andthe like.

The polyethylene-based resin is not particularly limited as far as it isnot a polyethylene-based resin obtained using a metallocene compoundcontaining a tetravalent transition metal as a polymerization catalyst,and examples thereof may include linear low density polyethylene, lowdensity polyethylene, intermediate density polyethylene, high densitypolyethylene, ethylene-a-olefin copolymer containing 50% by weight ormore of ethylene, ethylene-vinyl acetate copolymer containing 50% byweight or more of ethylene, and the like. These may be used alone, ortwo or more of them may be used in combination. Examples of α-olefinconstituting the ethylene-α-olefin-copolymer may include propylene,1-butene, 1-pentene, 4-methyl-1-pentene, 1-hexene, 1-heptene, 1-octeneand the like.

Examples of the polypropylene-based resin may include polypropylene, apropylene-α-olefin copolymer containing 50% by weight of propylene, andthe like. These may be used alone, or two or more of them may be used incombination. Examples of α-olefin constituting a propylene-α-olefincopolymer may include ethylene, 1-butene, 1-pentene, 4-methyl-1-pentene,1-hexene, 1-heptene, 1-octene and the like.

If a degree of crosslinking of the crosslinked polyolefin-based resinfoam sheet is small, upon stretching of a foam sheet, a cell at aportion in the vicinity of a surface of the foam sheet is broken, thesurface is roughened, and appearance of the resulting crosslinkedpolyolefin-based resin foam sheet is reduced. On the other hand, if thedegree of crosslinking is large, a melt viscosity of an expandablepolyolefin-based resin composition becomes too high, upon heating theexpandable polyolefin-based resin composition to foam it, the expandablepolyolefin-based resin composition becomes difficult to followexpansion, and a crosslinked polyolefin-based resin foam sheet having adesired expansion ratio can not be obtained. Therefore, the degree ofcrosslinking is limited to 5 to 60% by weight, preferably 10 to 40% byweight.

The degree of crosslinking of the crosslinked polyolefin-based resinfoam sheet refers to a degree of crosslinking measured by the followingmanner. About 100 mg of a test piece is taken from a crosslinkedpolyolefin-based resin foam sheet, and a weight A (mg) of the test pieceis weighed precisely. Then, this test piece is immersed in 30 cm³ ofxylene at 120° C., allowed to stand for 24 hours, filtered with a 200mesh wire gauze to take insolubles on the wire gauze, which is dried invacuum, and weight B (mg) of the insolubles is weighed precisely. Fromthe resulting value, a degree of crosslinking (% by weight) iscalculated from the following equation.Degree of crosslinking (% by weight)=100×(B/A)

Further, the crosslinked polyolefin-based resin foam sheet is requiredthat an aspect ratio of a cell (MD average cell diameter/CD average celldiameter) is 0.25 to 1, or an aspect ratio of a cell (CD average celldiameter[VD average cell diameter) is 2 to 18, and it is preferable thatan aspect ratio of a cell (MD average cell diameter/CD average celldiameter) is 0.25 to 1, and an aspect ratio of a cell (CD average celldiameter/VD average cell diameter) is 2 to 18.

More specifically, if a ratio of a MD average cell diameter and a CDaverage cell diameter in the crosslinked polyolefin-based resin foamsheet, that is, an aspect ratio of a cell (MD average cell diameter/CDaverage cell diameter) is small, an expansion ratio is reduced so thatflexibility is reduced, or a variation in a thickness, flexibility and atensile strength of the crosslinked polyolefin-based resin foam sheet isoccurred in some cases. On the other hand, if the ratio is large,flexibility of the crosslinked polyolefin-based resin foam sheet isreduced. Therefore, the ratio is preferably 0.25 to 1, more preferably0.25 to 0.60.

In addition, if a ratio of a CD average cell diameter and a VD averagecell diameter in the crosslinked polyolefin-based resin foam sheet, thatis, an aspect ratio of a cell (CD average cell diameter/VD average celldiameter) is small, flexibility of the crosslinked polyolefin-basedresin foam sheet is reduced. On the other hand, if the ratio is large,an expansion ratio is reduced so that flexibility is reduced, or avariation in a thickness, flexibility and a tensile strength of thecrosslinked polyolefin-based resin foam sheet is occurred in some cases.Therefore, the ratio is preferably 2 to 18, more preferably 2.5 to 15.

Herein, as shown in FIG. 1, MD (Machine Direction) of a crosslinkedpolyolefin-based resin foam sheet 1 refers to an extrusion direction, CD(Crossing Direction) of the crosslinked polyolefin-resin foam sheet 1refers to a direction orthogonal to the MD (Machine Direction) and alongwith a surface of the crosslinked polyolefin-based resin foam sheet 1,and VD (Vertical (thickness) Direction) of the crosslinkedpolyolefin-based resin foam sheet 1 refers to a direction orthogonal tothe surface of the crosslinked polyolefin-based resin foam sheet 1.

Next, a MD average cell diameter of the crosslinked polyolefin-basedresin foam sheet refers to a diameter measured by the following manner:a crosslinked polyolefin-based resin foam sheet is cut with a planeparallel with VD at an approximately central part in its CD over a fulllength.

Thereafter, a cut section of the crosslinked polyolefin-based resin foamsheet is magnified 60 times using a scanning electron microscope (SEM),and a photograph is taken so that a full length of VD of the crosslinkedpolyolefin-based resin foam sheet is contained.

A straight line which is 15 cm in a length on the resulting photograph(actual length is 2500 μm before magnification) is drawn on a partcorresponding to a central part of VD of the crosslinkedpolyolefin-based resin foam sheet so that it is parallel with a surfaceof the crosslinked polyolefin-based resin foam sheet.

Then, the number of cells positioned on the straight line is countedwith naked eyes, and a MD average cell diameter of a cell is calculatedbased on the following equation.MD average cell diameter (μm)=2500 (μm)/cell number

A VD average cell diameter of the crosslinked polyolefin-based resinfoam sheet refers to a diameter measured by the following manner: aphotograph is taken by the same manner as the manner upon calculation ofthe MD average cell diameter of the crosslinked polyolefin-based resinfoam sheet.

In the resulting photograph, three straight lines dividing a cut sectionof the photographed crosslinked polyolefin-based resin foam sheet intofour in MD are drawn in a direction orthogonal to a surface of thecrosslinked polyolefin-based resin foam sheet (VD) over a full length ofa foam sheet.

Thereafter, a length of each straight line is measured and, at the sametime, the number of cells positioned on each straight line is countedwith naked eyes, a VD average cell diameter of a cell is calculated foreach straight line based on the following equation, and an arithmeticmean thereof is regarded as a VD average cell diameter of a cell.VD average cell diameter (μm)=length of straight line on photograph(μm)/(60×cell number)

Next, a CD average cell diameter of the crosslinked polyolefin-basedresin foam sheet refers to a diameter measured by the following manner:the crosslinked polyolefin-based resin foam sheet is cut with a planeparallel with its CD and a direction orthogonal to a surface of thecrosslinked polyolefin-based resin foam sheet (VD) over a full length ina thickness direction.

Thereafter, a cut section of the crosslinked polyolefin-based resin foamsheet is magnified 60 times using a scanning electron microscope (SEM),and a photograph is taken so that a full length in a thickness directionof the crosslinked polyolefin-based resin foam sheet is contained.

Based on the resulting photograph, a CD average cell diameter iscalculated by the same manner as that upon measurement of the MD averagecell diameter of the crosslinked polyolefin-based resin foam sheet.

Upon counting of the number of cells positioned on a straight line inthe manner for measuring the aforementioned average cell diameter, acell diameter is determined based only on a cell cross section appearingon the photograph.

That is, there is a case where although cells appear completelyseparated by a cell wall on the cut section of the crosslinkedpolyolefin-based resin foam sheet, they are communicated at a portionother than the cut section of the crosslinked polyolefin-based resinfoam sheet. However, in the present invention, whether they arecommunicated or not at the portion other than the cut section of thecrosslinked polyolefin-based resin foam sheet is not taken intoconsideration, a cell morphology is determined based only on a crosssection of a cell appearing on the photograph, and one cavity portioncompletely surrounded by the cross section of the cell membraneappearing on the photograph is determined as one cell.

Positioning on a straight line refers to a case where the straight lineis completely penetrated at an arbitrary portion of the cell. Inaddition, at both ends of the straight line, in the case where an end ofthe straight line is situated in a cell without complete penetration ofa straight line through a cell, this cell was counted as 0.5.

Upon photographing of the cut section of the crosslinkedpolyolefin-based resin foam sheet, when the cut section of thecrosslinked polyolefin-based resin foam sheet is colored, discriminationof a cell becomes easy and, at the same time, when a scale of 2500 μm ismagnified simultaneously and is photographed, it becomes easy to specifya length of the straight line on the photograph.

In addition, if a 25% compression strength according to JIS K6767 of thecrosslinked polyolefin-based resin foam sheet is large, feeling of thecrosslinked polyolefin-based resin foam sheet is lowered, or impactabsorbability is reduced in some cases. Therefore, 4.9×10⁴ Pa or less ispreferable. If the strength is too small, upon binding of thecrosslinked polyolefin-based resin foam sheet, the sheet is crushed in athickness direction, and a thickness is reduced in some cases.Therefore, 2×10⁴ to 4×10⁴ Pa is more preferable.

Further, if a tensile strength at 23° C. in at least one direction of MDand CD in the crosslinked polyolefin-based resin foam sheet is small,when a crosslinked polyolefin-based resin foam sheet is used as a basematerial of an adhesive tape, there is a possibility that thecrosslinked polyolefin-based resin foam sheet is cut during use.Therefore, 1.96×10⁶ Pa or more is preferable. If the strength is toolarge, when the crosslinked polyolefin-based resin foam sheet is used asa base material of an adhesive tape, there is a possibility that itbecomes difficult to manually cut the adhesive tape, and handleabilityis reduced. Therefore, 2.2×10⁶ to 8.0×10⁶ Pa is more preferable.

A tensile strength at 23° C. in MD or CD of the crosslinkedpolyolefin-based resin foam sheet refers to a strength measuredaccording to JIS K6767.

In addition, if a thermal dimensional change rate at 90° C. of thecrosslinked polyolefin-based resin foam sheet is small, when thecrosslinked polyolefin-based resin foam sheet is used as a base materialof an adhesive tape, there is a possibility that when heat resistance ofthe crosslinked polyolefin-based resin foam sheet is reduced and heat isapplied, the adhesive tape is shrunk and is deviated from an applicationposition. Therefore, −10% or more is preferable. If the rate is toolarge, when the crosslinked polyolefin-based resin foam sheet is used asa base material of an adhesive tape, there is a possibility that, whenheat is applied to the crosslinked polyolefin-based resin foam sheet,the adhesive tape is inflated, and is deviated from an applicationposition. Therefore, −10 to 5% is more preferable, and −2.0 to 2.0% isparticularly preferable.

A thermal dimensional change rate at 90° C. of the crosslinkedpolyolefin-based resin foam sheet refers to a rate measured according toJIS K6767 except that a measuring temperature is 90° C.

Then, a process for producing the crosslinked polyolefin-based resinfoam sheet will be explained. The process for producing the crosslinkedpolyolefin-based resin foam sheet is not particularly limited, andexamples thereof may include (1) a process for producing a crosslinkedpolyolefin-based resin foam sheet, comprising the steps of: feeding apolyolefin-based resin containing 40% by weight or more of apolyethylene-based resin obtained using a metallocene compoundcontaining a tetravalent transition metal as a polymerization catalyst,and a thermally degradable blowing agent to an extruder, melting andkneading them, and extruding the kneaded material through the extruderinto a sheet to prepare an expandable polyolefin-based resin sheet;irradiating the expandable polyolefin-based resin sheet with ionizingradiation to crosslink the expandable polyolefin-based resin sheet so asto have a degree of crosslinking of 5 to 60% by weight; and heating andexpanding the crosslinked expandable polyolefin-based resin sheet, andstretching the resulting foam sheet towards any one or both ofdirections of MD and CD while maintaining the melted state at expansionto stretch a cell of the foam sheet, thereby producing a crosslinkedpolyolefin-based resin foam sheet in which an aspect ratio of a cell (MDaverage cell diameter/CD average cell diameter) is 0.25 to 1 or anaspect ratio of a cell (CD average cell diameter/VD average celldiameter) is 2 to 18, (2) a process for producing a crosslinkedpolyolefin-based resin foam sheet, comprising the steps of: feeding apolyolefin-based resin containing 40% by weight or more of apolyethylene-based resin obtained using a metallocene compoundcontaining a tetravalent transition metal as a polymerization catalyst,a thermally degradable blowing agent and an organic peroxide to anextruder, melting and kneading them, and extruding the kneaded materialthrough the extruder into a sheet to prepare an expandablepolyolefin-based resin sheet; heating the expandable polyolefin-basedresin sheet to decompose the organic peroxide, and expanding theexpandable polyolefin-based resin sheet while crosslinking it so as tohave a degree of crosslinking of 5 to 60% by weight; and stretching theresulting foam sheet towards any one or both of directions of MD and CDwhile maintaining the melted state at expansion to stretch a cell of thefoam sheet, thereby producing a crosslinked polyolefin-based resin foamsheet in which an aspect ratio of a cell (MD average cell diameter/CDaverage cell diameter) is 0.25 to 1 or an aspect ratio of a cell (CDaverage cell diameter/VD average cell diameter) is 2 to 18, (3) aprocess for producing a crosslinked polyolefin-based resin foam sheet,comprising the steps of: feeding a polyolefin-based resin containing 40%by weight or more of a polyethylene-based resin obtained using ametallocene compound containing a tetravalent transition metal as apolymerization catalyst, and a thermally degradable blowing agent to anextruder, melting and kneading them, and extruding the kneaded materialthrough the extruder into a sheet to prepare an expandablepolyolefin-based resin sheet; irradiating the expandablepolyolefin-based resin sheet with ionizing radiation to crosslink theexpandable polyolefin-based resin sheet so as to have a degree ofcrosslinking of 5 to 60% by weight; heating and expanding thecrosslinked expandable polyolefin-based resin sheet, and cooling thesheet to prepare a foam sheet; heating again the foam sheet into amelted state or a softened state; and stretching the foam sheet towardsany one or both of directions of MD and CD to stretch a cell of the foamsheet, thereby producing a crosslinked polyolefin-based resin foam sheetin which an aspect ratio of a cell (MD average cell diameter/CD averagecell diameter) is 0.25 to 1 or an aspect ratio of a cell (CD averagecell diameter/VD average cell diameter) is 2 to 18, (4) a process forproducing a crosslinked polyolefin-based resin foam sheet, comprisingthe steps of: feeding a polyolefin-based resin containing 40% by weightor more of a polyethylene-based resin obtained using a metallocenecompound containing a tetravalent transition metal as a polymerizationcatalyst, a thermally degradable blowing agent and an organic peroxideto an extruder, melting and kneading them, and extruding the kneadedmaterial through the extruder into a sheet to prepare an expandablepolyolefin-based resin sheet; heating the expandable polyolefin-basedresin sheet to decompose the organic peroxide, expanding the expandablepolyolefin-based resin sheet while crosslinking it so as to have adegree of crosslinking of 5 to 60% by weight, and cooling the sheet toprepare a foam sheet; heading again the foam sheet into a melted stateor a softened state; and stretching the foam sheet towards any one orboth of directions of MD and CD to stretch a cell of the foam sheet,thereby producing a crosslinked polyolefin-based resin foam sheet inwhich an aspect ratio of a cell (MD average cell diameter/CD averagecell diameter) is 0.25 to 1 or an aspect ratio of a cell (CD averagecell diameter/VD average cell diameter) is 2 to 18, and the like.

The thermally degradable blowing agent is not particularly limited asfar as it has been conventionally used for preparing a foam, andexamples thereof may include azodicarbonamide,N,N′-dinitrosopentamethylenetetramine, p-toluenesulfonylsemicarbazideand the like. Among these, azodicarbonamide is preferable. The thermallydegradable blowing agents may be used alone, or two or more of them maybe used in combination.

An amount of the thermally degradable blowing agent to be added in theexpandable polyolefin-based resin composition may be appropriatelydetermined depending on an expansion ratio of the crosslinkedpolyolefin-based resin foam sheet. If the amount is small, expandabilityof the expandable polyolefin-based resin sheet is reduced, and thecrosslinked polyolefin-based resin foam sheet having a desired expansionratio can not be obtained in some cases. On the other hand, if theamount is large, a tensile strength and compression recovering propertyof the resulting crosslinked polyolefin-based resin foam sheet arereduced in some cases. Therefore, the amount is preferably 1 to 40 partsby weight, more preferably 1 to 30 parts by weight with respect to 100parts by weight of a polyolefin-based resin.

If necessary, an antioxidant such as 2,6-di-t-butyl-p-cresol, anexpansion aid such zinc oxide, a cell nucleus adjusting agent, a thermalstabilizer, a coloring agent, a flame-retardant, an anti-static agent, afiller and the like may be added to the expandable polyolefin-basedresin composition in such a range that physical properties of thecrosslinked polyolefin-based resin foam sheet are not deteriorated.

Examples of a method of crosslinking the expandable polyolefin-basedresin sheet may include a method of irradiating an expandablepolyolefin-based resin sheet with ionizing radiation such as electronbeam, α-ray, β-ray and γ-ray, a method of blending an organic peroxidein an expandable polyolefin-based resin composition in advance, andheating the resulting expandable polyolefin-based resin sheet todecompose the organic peroxide, and the like. These methods may be usedin combination.

Examples of the organic peroxide may include1,1-bis(t-butylperoxy)3,3,5-trimethylcyclohexane,1,1-bis(t-butylperoxy)cyclohexane, 2,2-bis(t-butylperoxy)octane,n-butyl-4,4-bis(t-butylperoxy)valerate, di-t-butyl peroxide,t-butylcumyl peroxide, dicumyl peroxide,α,α′-bis(t-butylperoxy-m-isopropyl)benzene,2,5-dimethyl-2,5-di(t-butylperoxy)hexane,2,5-dimethyl-2,5-di(t-butylperoxy)hexyne-3, benzoyl peroxide,cumylperoxy neodecanate, t-butylperoxy benzoate,2,5-dimethyl-2,5-di(benzoylperoxy)hexane, t-butylperoxyisopropylcarbonate, t-butylperoxy allylcarbonate and the like. These maybe used alone, or two or more of them may be used in combination.

If an amount of the organic peroxide to be added is small, crosslinkingof the expandable polyolefin-based resin sheet becomes insufficient insome cases. On the other hand, if the amount is large, a degradationresidue of the organic peroxide remains in the resulting crosslinkedpolyolefin-based resin foam sheet in some cases. Therefore, the amountis preferably 0.01 to 5 parts by weight, more preferably 0.1 to 3 partsby weight, per 100 parts by weight a polyolefin-based resin.

In addition, a method of expanding the expandable polyolefin-based resinsheet is not particularly limited, and examples thereof may include amethod of heating by a hot air, a method of heating by infrared-ray, amethod by a salt bath, a method by an oil bath, and the like. These maybe used in combination.

Stretching of the foam sheet may be performed after the expandablepolyolefin-based resin sheet is expanded to obtain a foam sheet, or maybe performed while the expandable polyolefin-based resin sheet isexpanded. When the expandable polyolefin-based resin sheet is expandedto obtain a foam sheet, and the foam sheet is stretched, the foam sheetmay be stretched while the melted state at expansion is maintainedwithout cooling the foam sheet, or the foam sheet is cooled, the foamsheet is heated again into a melted state or a softened state,thereafter, the foam sheet may be stretched.

The melted state of the foam sheet refers to a state where the foamsheet is heated so that a temperature of both sides thereof becomes amelting point or more of a polyolefin-based resin constituting the foamsheet. A melting point (° C.) of the polyolefin-based resin refers to amaximum peak temperature among endothermic peaks accompanied withmelting of a crystal obtained when calorimetric analysis is performed bydifferential scanning calorimetry (DSC).

In addition, the softened sheet of the foam sheet refers to a statewhere the foam sheet is heated to a temperature so that a temperature T(° C.) of both sides thereof satisfies the following equation. Asoftening point (° C.) of the polyolefin-based resin refers to a vicatsoftening point measured based onASTM D1525.

Softening point of polyolefin-based resin −10° C.≦T≦softening point ofpolyolefin-based resin +10° C.

By stretching the foam sheet, a cell of the foam sheet is stretched anddeformed in a predetermined direction, so that a crosslinkedpolyolefin-based resin foam sheet having an aspect ratio of a cellwithin a predetermined range can be produced.

Further, regarding a stretching direction of the foam sheet, the sheetis stretched towards MD or CD, or towards MD and CD of a continuousexpandable polyolefin-based resin. When the expandable polyolefin-basedresin sheet is stretched towards MD and CD, the foam sheet may bestretched simultaneously towards MD and CD, or may be stretchedseparately every one direction.

Examples of a method of stretching the foam sheet in MD may include amethod of stretching the foam sheet in MD by rendering a speed ofwinding (winding speed) a continuous foam sheet while cooling afterexpansion faster than a speed of feeding (feeding speed) a continuousexpandable polyolefin-based resin sheet to an expanding step, a methodof stretching the foam sheet in MD by rendering a speed of winding(winding speed) the foam sheet faster than a speed of feeding (feedingspeed) the resulting foam sheet to a stretching step, and the like.

In the former method, since the expandable polyolefin-based resin sheetis inflated in MD by expansion of itself, when the foam sheet isstretched in MD, an inflated portion in MD of the expandablepolyolefin-based resin sheet by expansion is taken into consideration,and it is necessary to adjust a rate of feeding the sheet and a rate ofwinding the sheet so that the foam sheet is stretched in MD more thanthat inflated portion.

As a method of stretching the foam sheet in CD, a method of graspingboth ends of CD of the foam sheet with a pair of grasping members, andstretching the foam sheet in CD by moving gradually the pair of graspingmembers in such a direction that members are apart is preferable. Sincethe expandable polyolefin-based resin sheet is inflated in CD byexpansion of itself, when the foam sheet is stretched in CD, an inflatedportion in CD of the expandable polyolefin-based resin sheet byexpansion is taken into consideration, and it is necessary to adjust sothat the foam sheet is stretched in CD more than that inflated portion.

Herein, if a stretching ratio in MD of the crosslinked polyolefin-basedresin foam sheet is small, flexibility and a tensile strength of thecrosslinked polyolefin-based resin foam sheet are reduced in some cases.On the other hand, if the rate is large, the foam sheet is cut duringstretching, or an expanding gas is escaped from the foam sheet duringexpansion, an expansion ratio of the resulting crosslinkedpolyolefin-based resin foam sheet is remarkably reduced, flexibility anda tensile strength of the crosslinked polyolefin-based resin foam sheetare reduced, or quality becomes uneven in some cases. Therefore, theratio is preferably 1.1 to 2.0 times, more preferably 1.2 to 1.5 times.

A stretching ratio in MD of the crosslinked polyolefin-based resin foamsheet is calculated by the following manner. That is, a cubic root F ofan expansion ratio of the crosslinked polyolefin-based resin foam sheetis obtained, and a ratio of a winding speed and a feeding speed (windingspeed/feeding speed) V is obtained, so that a stretching ratio in MD ofthe crosslinked polyolefin-based resin foam sheet can be calculatedbased on the following equation. However, an expansion ratio of thecrosslinked polyolefin-based resin foam sheet refers to a specificgravity of the expandable polyolefin-based resin sheet divided by aspecific gravity of the crosslinked polyolefin-based resin foam sheet.Stretching rate (times) in MD of a foam sheet=V/F

If a stretching ratio in CD of the crosslinked polyolefin-based resinfoam sheet is small, flexibility and a tensile strength of thecrosslinked polyolefin-based resin foam sheet are reduced. On the otherhand, if the stretching ratio is large, the foam sheet is cut duringstretching, or an expansion gas is escaped from the foam sheet duringexpansion, an expansion ratio of the resulting crosslinkedpolyolefin-based resin foam sheet is remarkably reduced, flexibility anda tensile strength of the crosslinked polyolefin-based resin foam sheetare reduced, or quality becomes uneven. Therefore, the ratio ispreferably 1.2 to 4.5 times, more preferably 1.5 to 3.5 times.

A stretching ratio in CD in the crosslinked polyolefin-based resin foamsheet can be calculated based on the following equation when a length inCD of a crosslinked polyolefin-based resin foam sheet obtained byheating and expanding the expandable polyolefin-based resin sheetwithout stretching in its MD and CD is W₁, and a length in CD of thecrosslinked polyolefin-based resin foam sheet stretched in CD is W₂.Stretching rate (times) in CD of foam sheet=W ₂ /W ₁

Utility of the resulting crosslinked polyolefin-base resin foam sheet isnot particularly limited, and the sheet is used as an adhesive tape bylayering and integrating an adhesive layer on and with at least one sideof the crosslinked polyolefin-based resin foam sheet, or is used as amedical patch by applying a medicament on one side of the crosslinkedpolyolefin-based resin foam sheet.

More specifically, an adhesive tape formed by using the crosslinkedpolyolefin-based resin foam sheet as a base material is used as anadhesive tape for absorbing irregularities of an irregular plane, or isused as an electronic appliance sealing material for preventingapplication of impact on an electronic part mounted in an electronicappliance main body such as a mobile phone and a video camera, andpreventing entrance of a dust into the electronic appliance main body.In particular, since a thickness of the crosslinked polyolefin-basedresin foam sheet can be reduced while retaining excellent flexibilityand heat resistance, it can be suitably used in utility of an electronicappliance which has been remarkably miniaturized.

When the crosslinked polyolefin-based resin foam sheet is used as a basematerial of an adhesive tape, if the thickness of a crosslinkedpolyolefin-based resin foam sheet is small, flexibility and a tensilestrength of a crosslinked polyolefin-based resin foam sheet are reduced,and texture and a mechanical strength of the resulting adhesive tape arereduced. On the other hand, even if the thickness is large, improvementin performance of an adhesive tape can not be predicted, and economicalproperty is deteriorated. Therefore, the thickness is preferably 0.05 to2 mm, more preferably 0.1 to 8 mm.

In addition, an adhesive constituting an adhesive layer which is layeredon and integrated with one side or both sides of the crosslinkedpolyolefin-based resin foam sheet is not particularly limited as far asit has been conventionally used in an adhesive tape, and examplesthereof may include an acrylic-based adhesive, a urethane-basedadhesive, a rubber-based adhesive and the like.

Further, examples of a method of layering and integrating an adhesivelayer on and with the crosslinked polyolefin-based resin foam sheet byapplying the adhesive on at least one side of the crosslinkedpolyolefin-based resin foam sheet may include a method of applying anadhesive onto at least one side of a crosslinked polyolefin-based resinfoam sheet using a coating machine such as a coater, a method ofspraying and applying an adhesive onto at least one side of acrosslinked polyolefin-based resin foam sheet using a spray, a method ofapplying an adhesive onto at least one side of a crosslinkedpolyolefin-based resin foam sheet using a brush, and the like.

EFFECTS OF THE INVENTION

The crosslinked polyolefin-based resin foam sheet of the presentinvention is a crosslinked polyolefin-based resin foam sheet obtained byfeeding a polyolefin-based resin and a thermally degradable blowingagent to an extruder, melting and kneading them, and extruding thekneaded material through the extruder into a sheet to form an expandablepolyolefin-based resin sheet, and expanding the sheet, wherein a degreeof crosslinking of the crosslinked polyolefin-based resin foam sheet is5 to 60% by weight, an aspect ratio of a cell is within a predeterminedrange, and the polyolefin-based resin contains 40% by weight or more ofa polyethylene-based resin obtained using a metallocene compoundcontaining a tetravalent transition metal as a polymerization catalyst.Therefore, a thickness of the sheet can be reduced while maintainingexcellent flexibility and heat resistance.

In addition, when a 25% compression strength according to JIS K6767 is4.9 x 104 Pa or less, a tensile strength at 23° C. in at least onedirection of MD and CD is 1.96×10⁶ Pa or more, and a thermal dimensionalchange rate at 90° C. is −10% or more in the crosslinkedpolyolefin-based resin foam sheet, touch feeding, impact absorbingproperty and heat resistance are further excellent.

Further, since an adhesive tape in which an adhesive layer is layered onand integrated with one side of the crosslinked polyolefin-based resinfoam sheet having a thickness of 0.05 to 2 mm is excellent property ofabsorbing irregularities of an irregular plane and, also, can reduce athickness while retaining excellent flexibility and impact resistance,the tape can be also suitably used as a sealing material for protectingparts of a small electronic appliance.

A polyolefin-based resin constituting the crosslinked polyolefin-basedresin foam sheet of the present invention contains 40% by weight or moreof a polyethylene-based resin obtained using a metallocene compoundcontaining a tetravalent transition metal as a polymerization catalyst.

Although the polyethylene-based resin obtained using a metallocenecompound is flexible, the resin has little adherability; therefore, thepolyolefin-based resin containing 40% by weight or more of thispolyethylene-based resin exhibits little adherability in its meltedstate.

Therefore, when a crosslinked polyolefin-based resin foam sheet isproduced by stretching a foam sheet obtained by expanding thepolyolefin-based resin containing 40% by weight or more of apolyethylene-based resin obtained using a metallocene compoundcontaining a tetravalent transition metal as a polymerization catalyst,even if cell walls of the foam sheet are in the closed state, situationwhere cell walls are adhered to and integrated with one another can begenerally prevented; therefore, a crosslinked polyolefin-based resinfoam sheet having an aspect ratio of a cell within a desired range canbe obtained simply.

In addition, the polyethylene-based resin obtained using a metallocenecompound has a narrow molecular weight distribution and, in the case ofa copolymer, a copolymer component is introduced in any molecular weightcomponent at an appropriately equal ratio, and an expandablepolyolefin-based resin sheet can be generally crosslinked evenly.

Therefore, when a crosslinked polyolefin-based resin foam sheet isproduced by stretching a foam sheet, the foam sheet can be totallyevenly stretched, and the resulting crosslinked polyolefin-based resinfoam sheet has a generally even thickness and, at the same time, anaspect ratio of a cell is totally even, and quality such as mechanicalstrength and flexibility is totally even.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic view showing MD, CD and VD of a crosslinkedpolyolefin-based resin foam sheet.

EXPLANATION OF SYMBOLS

1: crosslinked polyolefin-based resin foam sheet

BEST MODE FOR CARRYING OUT THE INVENTION EXAMPLES 1 TO 4, COMPARATIVEEXAMPLE 4

An expandable polyolefin-based resin composition consisting of 100 partsby weight of linear low density polyethylene (made by Exxon ChemicalCompany, trade name: “EXACT3027”, density: 0.900g/cm³, weight-averagemolecular weight: 2.0, melting point: 98° C., softening point: 85° C.)obtained using a metallocene compound containing a tetravalenttransition metal as a polymerization catalyst, 5 parts by weight ofazodicarbonamide, 0.3 part by weight of 2,6-di-t-butyl-p-cresol and 1part by weight of zinc oxide was fed to an extruder, melted and kneadedat 130° C., and extruded into a continuous expandable polyolefin-basedresin sheet having a width of 200 mm and a thickness of 0.8 mm.

Then, both sides of the continuous expandable polyolefin-based resinfoam sheet were irradiated with an electron beam of an accelerationvoltage of 800 kV at 5 Mrad to crosslink the expandable polyolefin-basedresin sheet, and this expandable polyolefin-based resin sheet wascontinuously sent into an expanding furnace retained at 250° C. with ahot air and an infrared heater, followed by heating and expansion.

Thereafter, after the resulting foam sheet was continuously sent outfrom the expanding furnace, this foam sheet was stretched in its CD inthe state where a temperature of both sides of the foam sheet wasmaintained at 200 to 250° C. and, at the same time, the foam sheet waswound at a winding speed faster than a speed of sending the expandablepolyolefin-based resin sheet into the expansion furnace (feeding speed),thereby, the foam sheet was stretched in MD to stretch and deform a cellof the foam sheet in CD and MD, to obtain a crosslinked polyolefin-basedresin foam sheet having a width, a thickness, a degree of crosslinkingand an expansion ratio shown in Table 1. The winding speed of the foamsheet was adjusted while an inflation portion in MD by expansion of theexpandable polyolefin-based resin sheet itself was taken intoconsideration. In addition, a ratio of the winding speed and the feedingspeed of the foam sheet (winding speed/feeding speed), as well as astretching ratio of MD and CD of the crosslinked polyolefin-based resinfoam sheet are shown in Table 1.

In Comparative Example 4, since a stretching ratio of MD and CD of thefoam sheet was too great, an expansion gas was escaped, and an expansionratio of the resulting crosslinked polyolefin-based resin foam sheetresults in a low level of 5.2 times.

EXAMPLE 5

According to the same procedure as that of Example 1 except that anamount of azodicarbonamide to be added was 3 parts by weight in place of5 parts by weight, extrusion was performed so that a thickness of theexpandable polyolefin-based resin sheet was 0.32 mm, and a ratio of afeeding speed and a winding speed (feeding rate/winding rate) of thefoam sheet, as well as a width of CD in the crosslinked polyolefin-basedresin foam sheet was 1050 mm, a crosslinked polyolefin-based resin foamsheet was obtained.

EXAMPLE 6

An expandable polyolefin-based resin composition consisting of 100 partsby weight of linear low density polyethylene (made by Exxon ChemicalCompany, trade name: “EXACT3027”, density: 0.900 g/cm³, weight-averagemolecular weight: 2.0, melting point: 98° C., softening point: 85° C.)obtained using a metallocene compound containing a tetravalenttransition metal as a polymerization catalyst, 5 parts by weight ofazodicarbonamide, 0.3 part by weight of 2,6-di-t-butyl-p-cresol and 1part by weight of zinc oxide was fed to an extruder, and melted andkneaded it at 130° C., which was extruded into a continuous expandablepolyolefin-based resin sheet having a width of 200 mm and a thickness of0.8 mm.

Then, both sides of the continuous expandable polyolefin-based resinsheet were irradiated with an electron beam of an acceleration voltageof 800 kV at 5 Mrad to crosslink the expandable polyolefin-based resinsheet, this expandable polyolefin-based resin sheet was continuouslysent into an expansion furnace retained at 250° C. by a hot air and aninfrared heater, was heated and expanded, and was cooled to prepare acontinuous foam sheet, which was wound in a coil.

The resulting foam sheet was successively fed to a stretching step, thefoam sheet was heated so that a temperature of its both sides became110° C., the foam sheet was stretched in its CD and, at the same time,the foam sheet was stretched in MD by winding the foam sheet at awinding speed faster than a feeding speed of the foam sheet, to stretchand deform a cell of the foam sheet in CD and MD to obtain a crosslinkedpolyolefin-based resin foam sheet having a width, a thickness, a degreeof crosslinking and an expansion ratio shown in Table 1. In addition, aratio of a winding speed and a feeding speed of the foam sheet (windingspeed/feeding speed), as well as a stretching ratio of MD and CD of thecrosslinked polyolefin-based resin foam sheet are shown in Table 1.

COMPARATIVE EXAMPLE 1

According to the same procedure as that of Example 1 except that thefoam sheet was not stretched in CD, a crosslinked polyolefin-based resinfoam sheet was obtained.

COMPARATIVE EXAMPLE 2

According to the same procedure as that of Example 1 except that a widthof CD of the foam sheet became 2000 mm, a crosslinked polyolefin-basedresin foam sheet was tried to be prepared, but the foam sheet was cut inCD, and a crosslinked polyolefin-based resin foam sheet could not beobtained.

COMPARATIVE EXAMPLE 3

According to the same procedure as that of Example 2 except that apolyolefin-based resin consisting of 20 parts by weight of linear lowdensity polyethylene (made by Exxon Chemical Company, trade name:“EXACT3027”, density: 0.900 g/cm³, weight-average molecular weight: 2.0)obtained using a metallocene compound containing a tetravalenttransition metal as a polymerization catalyst, and 80 parts by weight ofa polyethylene resin (density: 0.923 g/cm³) was used in place of 100parts by weight of linear low density polyethylene (made by ExxonChemical Company, trade name: “EXACT3027”, density: 0.900 g/cm³,weight-average molecular weight: 2.0) obtained using a metallocenecompound containing a tetravalent transition metal as a polymerizationcatalyst, a crosslinked polyolefin-based resin foam sheet was tried tobe prepared, but the foam sheet was cut in CD in a step of stretchingthe foam sheet, and a crosslinked polyolefin-based resin foam sheetcould not be obtained.

COMPARATIVE EXAMPLE 5

According to the same procedure as that of Example 1 except that apolyolefin-based resin consisting of 20 parts by weight of linear lowdensity polyethylene (made by Exxon Chemical Company, trade name:“EXACT3027”, density: 0.900 g/cm³, weight-average molecular weight: 2.0)obtained using a metallocene compound containing a tetravalenttransition metal as a polymerization catalyst and 80 parts by weight ofan ethylene-vinyl acetate copolymer (ethylene content: 18% by weight)was used in place of 100 parts by weight of linear low densitypolyethylene (made by Exxon Chemical Company, trade name: “EXACT3027”,density: 0.900 g/cm³, weight-average molecular weight: 2.0) obtainedusing a metallocene compound containing a tetravalent transition metalas a polymerization catalyst, a crosslinked polyolefin-based resin foamsheet was prepared.

Average cell diameters of MD, CD and VD in the resulting crosslinkedpolyolefin-based resin foam sheet, an aspect ratio of a cell (MD averagecell diameter/CD average cell diameter) (expressed as “MD/CD” inTable 1) and an aspect ratio of a cell (CD average cell diameter[VDaverage cell diameter) (expressed as “CD/VD” in Table 1), a 25%compression strength according to JIS K6767, tensile strengths at 23° C.in MD and CD, as well as a thermal dimensional change rate in MD at 90°C. are shown in Table 1. TABLE 1 Example 1 Example 2 Example 3 Example 4Example 5 Example 6 Winding speed/ 2.50 2.50 3.10 4.30 2.15 2.50 feedingspeed Stretching MD 1.2 1.2 1.4 2.0 1.3 1.2 ratio CD 2.0 3.0 1.4 1.5 2.42.0 (times) Average MD 112 110 145 200 165 115 cell CD 210 330 142 150370 218 diameter VD 38 24 44 30 28 36 (μm) Aspect MD/CD 0.53 0.33 1.021.33 0.45 0.53 ratio CD/VD 5.53 13.75 3.23 5.00 13.21 6.06 Width (mm)860 1300 600 640 1050 860 Thickness (mm) 0.6 0.4 0.75 0.5 0.1 0.6 Degreeof 25 25 25 25 25 25 crosslinking (wt %) Expansion ratio 9.8 9.6 9.9 9.64.7 9.9 (times) 25% compression 3.92 2.94 4.90 3.92 2.94 3.88 strength(×10⁴ Pa) Tension MD 3.03 2.94 3.92 4.41 6.88 2.98 strength CD 2.94 3.922.45 2.50 6.96 2.90 (×10⁶Pa) Thermal MD −2.0 −0.5 −3.0 −3.5 −3.7 −2.0dimensional CD −5.0 −8.0 −3.0 −3.5 −9.5 −7.0 change rate (%) ComparativeComparative Comparative Comparative Comparative Example 1 Example 2Example 3 Example 4 Example 5 Winding speed/ 2.50 2.50 2.50 4.10 2.50feeding speed Stretching MD 1.2 1.2 1.2 2.4 1.2 ratio CD 1.0 — — 5.8 2.0(times) Average MD 115 — — 190 305 cell CD 100 — — 310 300 diameter VD78 — — 15 48 (μm) Aspect MD/CD 1.15 — — 0.61 1.02 ratio CD/VD 1.28 — —20.67 6.25 Width (mm) 430 — — 1300 860 Thickness (mm) 1.2 — — 0.25 0.6Degree of 25 — — 25 25 crosslinking (wt %) Expansion ratio 10.0 — — 5.29.9 (times) 25% compression 9.8 — — 5.88 2.94 strength (×10⁴ Pa) TensionMD 2.99 — — 5.88 1.27 strength CD 2.06 — — 4.90 0.69 (×10⁶Pa) Thermal MD−3.0 — — −6.0 −20 dimensional CD −1.0 — — −8.0 −40 change rate (%)

INDUSTRIAL APPLICABILITY

The crosslinked polyolefin-based resin foam sheet of the presentinvention is suitable for using as an adhesive tape wherein an adhesivelayer is layered on and integrated with at least one side of thecrosslinked polyolefin-based resin foam sheet, or using as a medicalpatch wherein a medicament is applied on one side of the crosslinkedpolyolefin-based resin foam sheet.

1. A crosslinked polyolefin-based resin foam sheet obtained by feeding apolyolefin-based resin and a thermally degradable blowing agent to anextruder, melting and kneading them, extruding the kneaded materialthrough the extruder into a sheet to form an expandable polyolefin-basedresin sheet, and expanding the sheet, the crosslinked polyolefin-basedresin foam sheet comprising the polyolefin-based resin which contains40% by weight or more of a polyethylene-based resin obtained using ametallocene compound containing a tetravalent transition metal as apolymerization catalyst, the crosslinked polyolefin-based resin foamsheet having a degree of crosslinking of the crosslinkedpolyolefin-based resin foam sheet is of 5 to 60% by weight and an aspectratio of a cell (MD average cell diameter/CD average cell diameter) of0.25 to
 1. 2. The crosslinked polyolefin-based resin foam sheetaccording to claim 1, wherein an aspect ratio of a cell (CD average celldiameter/VD average cell diameter) is 2 to
 18. 3. A crosslinkedpolyolefin-based resin foam sheet obtained by feeding a polyolefin-basedresin and a thermally degradable blowing agent to an extruder, meltingand kneading them, extruding the kneaded material through the extruderinto a sheet to form an expandable polyolefin-based resin sheet, andexpanding the sheet, the crosslinked polyolefin-based resin foam sheetcomprising the polyolefin-based resin which contains 40% by weight ormore of a polyethylene-based resin obtained using a metallocene compoundcontaining a tetravalent transition metal as a polymerization catalyst,the crosslinked polyolefin-based resin foam sheet having wherein adegree of crosslinking of the crosslinked polyolefin-based resin foamsheet is of 5 to 60% by weight[[,]] and an aspect ratio of a cell (CDaverage cell diameter/VD average cell diameter) of 2 to
 18. 4. Thecrosslinked polyolefin-based resin foam sheet according to claim 1,wherein a 25% compression strength according to JIS K6767 is 4.9×10⁴ Paor less, a tensile strength at 23° C. in at least one direction. of MDand CD is 1.96×10⁶ Pa or more, and a thermal dimensional change rate ofMD at 90° C. is −10% or more.
 5. The crosslinked polyolefin-based resinfoam sheet according to claim 3, wherein a 25% compression strengthaccording to JIS K6767 is 4.9×10⁴ Pa or less, a tensile strength at 23°C. in at least one direction of MD and CD is 1.96×10⁶ Pa or more, and athermal dimensional change rate at 90° C. is −10% or more.
 6. Anadhesive tape comprising a crosslinked polyolefin-based resin foam sheethaving a thickness of 0.05 to 2 mm, and an adhesive layer which islayered on and integrated with at least one side of the crosslinkedpolyolefin-based resin foam sheet, wherein the crosslinkedpolyolefin-based resin foam sheet is obtained by feeding apolyolefin-based resin and a thermally degradable blowing agent to anextruder, melting and kneading them, extruding the kneaded materialthrough the extruder into a sheet to form an expandable polyolefin-basedresin sheet, and expanding the sheet, a degree of crosslinking of thecrosslinked polyolefin-based resin foam sheet is 5 to 60% by weight, anaspect ratio of a cell (MD average cell diameter/CD average celldiameter) is 0.25 to 1, and the polyolefin-based resin contains 40% byweight or more of a polyethylene-based resin obtained using ametallocene compound containing a tetravalent transition metal as apolymerization catalyst.
 7. An adhesive tape comprising a crosslinkedpolyolefin-based resin foam sheet having a thickness of 0.05 to 2 mm,and an adhesive layer which is layered on and integrated with at leastone side of the crosslinked polyolefin-based resin foam sheet, whereinthe crosslinked polyolefin-based resin foam sheet is obtained by feedinga polyolefin-based resin and a thermally degradable blowing agent to anextruder, melting and kneading them, extruding the kneaded materialthrough the extruder into a sheet to form an expandable polyolefin-basedresin sheet, and expanding the sheet, a degree of crosslinking of thecrosslinked polyolefin-based resin foam sheet is 5 to 60% by weight, anaspect ratio of a cell (CD average cell diameter/VD average celldiameter) is 2 to 18, and the polyolefin-based resin contains 40% byweight or more of a polyethylene-based resin obtained using ametallocene compound containing a tetravalent transition metal as apolymerization catalyst.
 8. A process for producing a crosslinkedpolyolefin-based resin foam sheet, comprising the steps of: feeding apolyolefin-based resin containing 40% by weight or more of apolyethylene-based resin obtained using a metallocene compoundcontaining a tetravalent transition metal as a polymerization catalyst,and a thermally degradable blowing agent to an extruder, melting andkneading them, and extruding the kneaded material through the extruderinto a sheet to prepare an expandable polyolefin-based resin sheet;crosslinking the expandable polyolefin-based resin sheet so as to have adegree of crosslinking of 5 to 60% by weight; and heating and expandingthe crosslinked expandable polyolefin-based resin sheet, and stretchingthe resulting foam sheet towards any one or both of directions of MD andCD while maintaining the melted state at expansion to stretch a cell ofthe foam sheet, thereby producing a crosslinked polyolefin-based resinfoam sheet in which an aspect ratio of a cell (MD average celldiameter/CD average cell diameter) is 0.25 to 1 or an aspect ratio of acell (CD average cell diameter/VD average cell diameter) is 2 to
 18. 9.A process for producing a crosslinked polyolefin-based resin foam sheet,comprising the steps of: feeding a polyolefin-based resin containing 40%by weight or more of a polyethylene-based resin obtained using ametallocene compound containing a tetravalent transition metal as apolymerization catalyst, and a thermally degradable blowing agent to anextruder, melting and kneading them, and extruding the kneaded materialthrough the extruder into a sheet to prepare an expandablepolyolefin-based resin sheet; heating the expandable polyolefin-basedresin sheet to expand the sheet while crosslinking it so as to have adegree of crosslinking of 5 to 60% by weight; and stretching theresulting foam sheet towards any one or both of directions of MD and CDwhile maintaining the melted state at expansion to stretch a cell of thefoam sheet, thereby producing a crosslinked polyolefin-based resin foamsheet in which an aspect ratio of a cell (MD average cell diameter/CDaverage cell diameter) is 0.25 to 1 or an aspect ratio of a cell (CDaverage cell diameter/VD average cell diameter) is 2 to
 18. 10. Aprocess for producing a crosslinked polyolefin-based resin foam sheet,comprising the steps of: feeding a polyolefin-based resin containing 40%by weight or more of a polyethylene-based resin obtained using ametallocene compound containing a tetravalent transition metal as apolymerization catalyst, and a thermally degradable blowing agent to anextruder, melting and kneading them, and extruding the kneaded materialthrough the extruder into a sheet to prepare an expandablepolyolefin-based resin sheet; crosslinking the expandablepolyolefin-based resin sheet so as to have a degree of crosslinking of 5to 60% by weight; heating and expanding the crosslinked expandablepolyolefin-based resin sheet to prepare a foam sheet; heating this foamsheet to obtain the melted or softened state; and stretching the foamsheet towards any one or both of directions of MD and CD to stretch acell of the foam sheet, thereby producing a crosslinked polyolefin-basedresin foam sheet in which an aspect ratio of a cell (MD average celldiameter/CD average cell diameter) is 0.25 to 1 or an aspect ratio of acell (CD average cell diameter/VD average cell diameter) is 2 to
 18. 11.A process for producing a crosslinked polyolefin-based resin foam sheet,comprising the steps of: feeding a polyolefin-based resin containing 40%by weight or more of a polyethylene-based resin obtained using ametallocene compound containing a tetravalent transition metal as apolymerization catalyst, and a thermally degradable blowing agent to anextruder, melting and kneading them, and extruding the kneaded materialthrough the extruder into a sheet to prepare an expandablepolyolefin-based resin sheet; heating the expandable polyolefin-basedresin sheet to expand the sheet while crosslinking it so as to have adegree of crosslinking of 5 to 60% by weight to prepare a foam sheet;heating the foam sheet to obtain the melted or softened state; andstretching the foam sheet towards any one or both of directions of MDand CD to stretch a cell of the foam sheet, thereby producing acrosslinked polyolefin-based resin foam sheet in which an aspect ratioof a cell (MD average cell diameter/CD average cell diameter) is 0.25 to1 or an aspect ratio of a cell (CD average cell diameter/VD average celldiameter) is 2 to 18.